1. Field of the Invention
This invention relates to a malfunction detecting system which detects malfunctions of a servo system and the like in a numerical control unit to provide for enhanced reliability in numerical control.
2. Description of the Prior Art
A numerical control unit has an interpolation function for generating command pulses in response to a distribution command and includes a servo system responsive to the command pulses to drive a movable part of a machine.
By way of example, a servo system has an error register supplied with command pulses and feedback pulses to store difference therebetween. A servo motor is driven so that the content of the error register is reduced to zero. In this servo system, during emission of the command pulses, the content of the error register assumes a certain value; but when the delivery of the command pulse is interrupted or completed by turning the distribution command OFF, the value of the error register gradually decreases to become smaller than a certain value .epsilon..sub.0 after a certain period of time .tau..
In the following cases, however, the content of the error register may sometimes be larger than the value .epsilon..sub.0 at a time .tau. after the distribution on command is turned OFF:
(1) Error register failure: A certain bit or bits remain "1"
(2) Interpolator failure: In spite of the absence of a distribution command, a command pulse is generated.
(3) Speed control servo system failure: In spite the absence of a move command, the servo motor is driven to yield feedback pulses.
In another example of a conventional servo system the phase difference between a command phase signal and a feedback phase signal is used to drive the servo motor so that to reduce the phase difference to zero. Also in such a servo system, the phase difference signal occurring at time .tau. after the distribution command is turned OFF may be in some cases larger than the certain value .epsilon..sub.0 owing to the aforesaid failures in the respective parts.